Energy generation and storage has long been a subject of study and development. Of special importance is the storage of electrical energy in a compact form that can be readily charged and discharged such as rechargeable batteries and/or electrochemical capacitors. High power, high current pulse rechargeable electrochemical charge storage devices, are very important in applications using electrical pulses, such as digital communications, power tools, and portable computers, to name but a few applications. In these devices, high electrochemical kinetic rate, long cycle life of the electrode, and good ionic conductivity of the electrolyte are all extremely important considerations.
Most high powered electrochemical charge storage devices, such as electrochemical capacitors, use aqueous electrolytes since aqueous electrolytes are known to have the highest ionic conductivities. However, aqueous electrolytes result in problems when handling and packaging the devices. This is due to the fact that aqueous electrolytes are generally liquid and hence have a propensity to leak out of the packages into which they are sealed. Furthermore, many high power electrochemical energy storage devices demonstrate poor cycle life due to dendrite growth on the electrodes thereof. Dendrite growth results in the formation of pinholes between the electrodes, thus resulting in short circuits which significantly reduce the cycle life of the battery, and which represent a severe limitation in the state of the conventional art.
Electrochemical capacitors can generally be divided into two subcategories: Double layer capacitors in which the interfacial capacitance at the electrode/electrolyte interface can be modeled as two parallel sheets of charge; and pseudocapacitor devices in which charge transfer between the electrode and the electrolyte occurs over a wide potential range. These charge transfers are the result of primarily, secondary, and tertiary oxidations/reduction reactions between the electrode and the electrolyte. These types of electrolyte capacitors are now being developed for high-pulse power applications, such as those described hereinabove.
Pseudocapacitor devices are disclosed in, for example, European Patent Application No. 82109061.0 to Dwight Craig. The devices disclosed and claimed in the Craig reference generally relate to pseudocapacitor devices having aqueous electrolytes therein. As such, the devices may be subject to the limitations of aqueous based systems such as those described hereinabove.
Furthermore, these devices suffer from high material cost and low cell voltage. There are two kinds of pseudocapacitor materials: metal oxides, i.e., (RuO.sub.2, IrO.sub.2, CoO.sub.2, etc.) and redox conductive polymers (i.e., polyaniline, polypyrrole, and polythiophene, etc.) Metal oxide capacitors are very expensive as many of the preferred metals, such as Ru and Ir, are very costly. Redox polymers have relatively high energy storage capacity, low cost and long cycle life. However, these conductive polymers have a narrow working voltage in proton conducting electrolytes. For example, a single cell device made from symmetric (i.e., two) polyaniline electrodes can only have 0.5 V device voltage. This drawback decreases the polymer device energy density significantly.
Accordingly, there exists a need to provide novel electrochemical capacitor devices free of the limitations inherent to prior art systems. Such devices should have high ionic conductivities, provide high power and high energy, and be fabricated of relatively environmentally benign materials. Moreover, fabrications of such devices should be simple, inexpensive, and readily repeatable.